1. Field of the Invention
This invention relates to a light emitting semiconductor device having an optical element, and more particularly to an improved light emitting semiconductor device having a light emitting element, such as a semiconductor laser element, a light emitting diode or the like, and an optical element such as a Fresnel lens or the like condense, collimate or diverge a ray emitted from the light emitting element, both elements of which are packaged as a single unit.
2. Discussion of the Related Art
In FIG. 19, there is shown a conventional light emitting element member A including a disk-shaped stem 1 made of metal, a rectangle metal projecting portion 2 formed on a center of a surface of the stem 1, a light emitting element 3 represented by a semiconductor laser element or the like which is mounted on a side wall of projecting portion 2 through a heat sink 4, a light receiving element 5 for monitoring the output of a ray emitted from the light emitting element 3 to control the output thereof, a metal cap 6 mounted on the stem 1 to cover the elements 3 and 5, and signal and ground terminals 7a through 7c. The pair of signal terminals 7a and 7b are mounted through the stem 1 in an insulated relationship to be electrically connected to light emitting element 3 and light receiving element 5 through wires 8. The ground terminal 7c is planted to stem 1 to be electrically connected thereto and further to elements 3 and 5.
This package type light emitting element member A is conventionally mass-produced and marketed at a reduced price, but has the disadvantages that, when it is employed as an optical pickup or sensor, it must be combined with an optical element such as a Fresnel lens to condense or collimate the diverging emitted light, so that many number of components are assembled and an optical axis is hard to be adjusted.
As illustrated in FIG. 20, the inventors have proposed a light emitting semiconductor device B further including an optical element 9 employing a Fresnel lens for collimating the light emitted from light emitting element 3, in addition to the construction of FIG. 19. A lens base plate 10 has a Fresnel lens at its central position, and its flat portion at a periphery of the lens is bonded to a top of the projecting portion 2 by bond. The distance between lens 9 and light emitting element 3 is adjusted by the thickness of an ultraviolet rays (UV) setting bond for securing the plate 10 to the projecting portion 2.
The light emitting semiconductor device of FIG. 20, however, still has the disadvantages that a focal position of a ray emitted from the light emitting element which has been thus adjusted to a predetermined position is varied by changes of temperatures of circumstances or the light emitting element itself, so that the focal distance of the device is unstable against temperatures. Such a poor temperature stability of the focal distance is caused by several reasons.
The primary reason for unstable focal distance is that the projecting portion employed by the light semiconductor device of FIG. 20 for supporting the lens base plate is made of metal having a large coefficient of thermal expansion, so that the distance between the laser chip and the Fresnel lens is delicately changed and the focal position of the ray emitted from the light emitting element is also changed.
The second reason is a change of a luminous wavelength by temperature of the semiconductor laser element. As shown in FIG. 21 illustrating the relation between luminous wavelength L and temperature T of a semiconductor laser element having a central wavelength 780 nm, the luminous wavelength L of the semiconductor laser element is varied by the temperature T of the element itself. The luminous wavelength L is shifted toward a long wavelength as the temperature T rises, but becomes shorter as the temperature T drops. A light emitting diode also has a similar luminous versus temperature curve in which its luminous wavelength becomes longer as a temperature of the diode rises. As shown in FIGS. 22(a) and (b), the Fresnel lens 9 is a kind of diffraction lens divided into a large number of ring-shaped lenses formed by lens materials, and generally mounted on a lens mounting base plate 10 at its surface. The Fresnel lens 9 provides a similar lens function to a conventional refractor with respect to rays emitted from a light emitting element represented by a semiconductor laser element or a light emitting diode as illustrated in FIGS. 22(a ) and (b) exemplarily showing a refracted ray. The lens can be formed of a miniaturized flat and thin plate, so that it plays as a short focus lens suitable to mounting and integrating use, which is easy to be manufactured by a molding process and has a reduced wave surface aberration. Accordingly, the Fresnel lens 9 can be used as a micro Fresnel lens in combination with a light emitting element employing a semiconductor laser element or a light emitting diode.
Since Fresnel lens is a lens utilizing diffraction phenomenon, a diffraction angle of a ray passing through Fresnel lens varies as an incident wavelength L varies, so that the focal position or distance F is fairly changed by the incident wavelength L. The focal distance F versus incident wavelength L is expressed by the following equation: EQU F=(Lo/L).multidot.Fo (1)
L: incident wavelength PA1 F: focal distance of incident ray PA1 Lo: designed wavelength of Fresnel lens PA1 Fo: designed focal distance of Fresnel lens
When a change of focal distance F is expressed by "f" as incident wavelength L is changed by "1", the change f is expressed by the following equation based on the above equation (1): EQU f=-f.multidot.l/L (2)
FIG. 23 shows a curve representing the relation between focal distance of Fresnel lens F and incident wavelength L expressed by the equation (1), in which the focal distance F is shortened as the incident wavelength L becomes longer but lengthened as the wavelength L becomes shorter.
Thus, even if a light emitting semiconductor device having a light emitting element in combination with Fresnel lens is designed to employ a Fresnel lens having the designed focal distance Fo versus the ray having a wavelength Lo emitted from the light emitting element, the focal distance L of Fresnel lens varies as the wavelength L of ray emitted from the light emitting element is varied by change of temperature T (hereinafter described as "thermal unstableness of focal distance caused by wavelength change of light emitting element"). For example, as shown in FIG. 24, the focal distance F of Fresnel lens is shortened as the temperature T of the light emitting semiconductor device rises while it is lengthened as the temperature T drops.
Thus, such a proposed light emitting semiconductor device is subject to change by temperatures of distance between an light emitting element and an optical element or thermal unstableness of wavelength of light emitting element, so that it has the disadvantages that focal distance or angle of emitting light is changed by temperature. The light emitting semiconductor device had the disadvantages that it cannot provide perfect collimated rays due to temperature change when it is employed as a collimated light source, the focal position is changed by temperature when the light emitting semiconductor device is employed as a condensed light source, and the angle of divergent is changed by temperature when the light emitting semiconductor device is employed as a diverged light source.